Forum software such as vbulletin offers modules such as 'undisposable' that block users of free email services including hushmail, hotmail, and whatever domains you specify.
This is usually an attempt to reduce spam, because free and disposable email addresses are more likely to be used by spammers.
To lift these restrictions, you need to determine where in your forum/content management system they're being applied. You'll probably need the highest level admin access in order to change settings like these. If you edit your question to explain what software you're using and how much control you have over it, we may be able to find instructions to override the restrictions or whitelist hushmail accounts for you.